GEOLOGY. 309 



these valleys during emergence in the way that took place in the passes of 

 Llanbcris and ISTant-Francon, and in parts of the Highlands of Scotland. It 

 has been stated above that the upper plain around Lake Erie, and the lower 

 plain of Lake Ontario, are alike covered with drift. Part of this was formed, 

 and much of it modified, during the emergence of the country. In the val- 

 ley of the St. Lawrence, near Montreal, about 100 feet above the river, there 

 are beds of clay, containing Leda Portlandica, and called by Dr. Dawson, of 

 Montreal, the Leda-clay. Dr. Dawson is of opinion, that, when this clay was 

 formed, the sea in which it was deposited washed the base of the old coast 

 line that now makes the great escarpment at Queenstown and Lewiston 

 overlooking the plains around Lake Ontario. It has long been an accepted 

 belief that the Falls of Niagara commenced at the edge of this escarpment, 

 and that the gorge has gradually been produced by the river wearing its way 

 back for seven miles to the place of the present Falls. In this case, the 

 author conceives that the Falls commenced during the deposition of the Leda-clay } 

 or near the close of the drift period, when, during the emergence of the coun- 

 try, the escarpment had already risen partly above water. 



If the 35,000 years suggested by Sir C- Lyell as the minimum for the time 

 occupied in the erosion of the gorge of Niagara be approximately correct, 

 though probably below the reality, we have an idea of the amount of time 

 that has elapsed since the close of the drift-period. And, if it be ever found 

 possible to accurately determine the ancient rate of recession, we shall have 

 data for a first approach to an actual measurement of a portion of geologi- 

 cal time. 



GEOLOGICAL STRUCTURE OF SOUTHERN AFRICA. 



Livingston's theory of the structure of the southern part of the African Con- 

 tinent is essentially new to the great body of naturalists, misled, as they have 

 been since the beginning of the century, by the huge blotch of red paint by 

 which geologists have chosen to represent a supposed central plateau of lava, 

 in correspondence with a similar formation in the centre of the Peninsula of 

 British India. It turns out that this is a mere analogy, a fiction and mistake. 

 Murchison developed, in his anniversary speech of 1852, from a study of 

 Bain's map, the same idea which Livingston obtained by his own experience 

 on the ground. Travelling among large Cape heaths, rhododendrons and Al- 

 pine roses, the botanist felt himself moving over a high table land, although 

 under a tropical sun. Descending suddenly from the centre of Africa five 

 thousand feet into the land of Cassange, watered by the river Quango, 

 guided in his estimate of depth by the rude method of plunging his ther- 

 mometer into boiling water, noticing the thin red strata of mud-rocks to lie 

 nearly horizontal, and remembering the enormous shallow lakes and laby- 

 rinth of mighty rivers forming the Zambesi and issuing through a deep 

 gorge upon the Indian coast, he thought he saw beneath him a recently and 

 slowly uplifted continent, of platter shape, with broken edges on the east 

 and west. The great north and south valley of Congo River cuts this conti- 

 nental plateau to its base. Mounting the opposite or western wall of the 

 valley, and crossing the western division of the plateau, he descended upon 

 the Loango coast, over the upturned edges of the rocks. Ho saw at once 

 why the long western coast of Africa is so straight. Like that of South 

 America, it runs along a deep geological break in the earth's crust, but 

 unaccompanied by erupted Andes ajid volcanic cones. It seems, however, 



